1. Field of the Invention
This invention relates to a method and apparatus for the manufacture of powder-filled steel tube from steel strip, wherein the strip, while continuously moving along a path, is formed into a channel, is filled with powder and is closed into a tube.
2. Description of the Prior Art
A method of powder-filled tube manufacture as described above is known from EP-A-O No. 158 693. In this, the open channel formed from the steel strip to a feeding station where it is filled with powder. The feeding station is equipped with two powder feeding systems mounted in tandem by means of which a ferromagnetic and a non-ferromagnetic powder are fed successively into the open channel. Subsequently, the open channel is closed by rolling between a set of shaped rolls downstream of the feeding station, and then sealed by welding in a welding station. In this method, on the one hand a cold air current arises over the moving open channel; this air current is generated by the movement of the strip and moves in the same direction, and entrains powder from the channel. On the other hand, there arises a warm air current flowing back from the closed tube; this warm air current results from the reduction in cross-section of the tube and is heated by the welding process and entrains powder from the filled tube.
The powder is poured into the open channel from a significant height, in order to obtain a higher density of the layer of powder in the open channel. By this filling method, known as gravimetric filling, especially the finest powder particles will be brought into a suspended condition, and are then readily entrained and dispersed into the surroundings by the air currents.
Powder particles entrained by air currents and powder particles whirling up cause problems in the manufacture of powder-filled steel tubes. One of the problems is that powder particles which have been temporarily entrained and powder particles which have whirled up deposit on the open channel, settling on those parts of the edge of the steel strip which are joined together when the tube is formed, the so-called strip edges, thereby affecting adversely the quality of the joint made subsequently. In EP-A-O 158 693 it is suggested to solve this problem of fouled strip edges by providing an air exhaust arrangement between the feeding station and the welding or folding station, for removal of the cold and warm air flows. The mouths of this exhaust arrangement are positioned over the open channel.